Belt press as a process of placing a belt-shaped target object on a press belt and pressing the target object between one pressing member located on the inner periphery of the press belt and the other pressing member located on the outer periphery of the press belt is used in various industries. As used herein, the “pressing member” refers to a press roll, a pressure shoe, etc. An example of the belt press is shoe press as dehydrating press in the papermaking industry.
The shoe press that is used in the papermaking industry will be briefly described as an example. The shoe press is a method of pressing (dehydrating) a target object (wet paper web) by placing the target object on the outer peripheral surface of a press belt and applying through the press belt a surface pressure to the target object between a press roll as external pressing means located on the outer periphery of the press belt and a pressure shoe as internal pressing means located on the inner periphery of the press belt. While roll press as press using two rolls applies a linear pressure to the target object, the shoe press can apply a surface pressure to the target object by using the pressure shoe having a predetermined width in a traveling direction. Accordingly, performing the dehydrating press by the shoe press is advantageous in that a nip width can be increased and hydrating efficiency can be enhanced.
In order to make the shoe press compact, shoe press rolls assembled in a roll shape by covering a pressure shoe as internal pressing means with a flexible cylindrical press belt (press jacket), as disclosed in, e.g., Japanese Unexamined Patent Application Publication No. S61-179359 (Patent Literature 1), have been widely used in the art.
In, e.g., the paper industry, the magnetic recording medium manufacturing industry, the textile industry etc., in order to improve quality of the target object, the shoe press may also be performed instead of the roll press or together with the roll press in processes other than the above dehydration process, such as a calendering process that is performed to make the surface of a target object smooth and glossy. Characteristics that are commonly required for the press belts include strength, abrasion resistance, flexibility, and impermeability to water, oil, gas, etc. Polyurethane that is obtained by causing reaction between urethane prepolymer and a curing agent is commonly used for the press belts as a material having these characteristics. However, since the press belts, in particular shoe press belts, are repeatedly subjected to severe bending and pressing, cracking tends to occur in the outer peripheral surfaces of the press belts, causing a serious problem in terms of durability. In particular, pressing means such as a shoe has inflection points at its ends where the pressure is released. Thus, the press belt portions located in the ends tend to be intensively subjected to bending and stress, and thus are susceptible to cracking.
In Japanese Unexamined Patent Application Publication No. 2005-97806 (Patent Literature 1), the applicant proposed a press belt having a shape designed in view of the above problems. This press belt includes end-corresponding regions corresponding to both ends in a lateral direction of pressing means and having a small thickness, and a central region located between the end-corresponding regions and having a larger thickness than the end-corresponding regions. The end-corresponding regions of the press belt are subjected to stress in longitudinal and lateral directions when in use, and thus are subjected to torsional stress. According to this improved press belt, the thickness of the end-corresponding regions is reduced to increase flexibility of these regions. Thus, when subjected to torsional stress, the end-corresponding regions absorb the torsional stress by flexural deformation, whereby cracking can be effectively suppressed.